Recently, various water-absorbent resins have been developed (for example, see U.S. Pat. No. 4,340,706, U.S. Pat. No. 4,093,776, U.S. Pat. No. 4,459,396, U.S. Pat. No. 3,980,663 and U.S. Pat. No. 4,552,938) and have been extensively used in various water-absorbent articles such as diaper, menstrual articles and the like. Thereby, properties of water-absorbent articles are improved. For example, liquid retention characteristics are improved and liquid leak is diminished. Further, a comfortable feeling in wearing water-absorbent article which has absorbed body fluids is achieved. Accordingly, such a water-absorbent article tends to be kept on for a longer period of time.
On the other hand, it has been found that a gel resulted from absorption of fluids such as urine, catamenial blood, secretions and the like by a water-absorbent article is crumbled to be out of shape because, in general, a water-absorbent resin of the gel is decomposed by the body fluids with time. Accordingly, as a period of time for keeping a water-absorbent article on becomes larger, liquid retention characteristics of the article are lowered, which results in increase in liquid leak, impairment of feeling in wearing the article and the like. In view of this, it is required to develop a water-absorbent resin which has an improved gel stability to the body fluids.
As a means for improving a gel stability, for example, increase in crosslinking density of a water-absorbent resin may be employed. However, as crosslinking density becomes higher, a water-absorption capacity becomes lower and therefore it is necessary to increase an amount of the resin to be used. This is less economical and causes such a disadvantage that properties of a water-absorbent article are adversely affected.